Introduction of COD national standard measurement method (what problems should be paid attention to in the determination of COD by national standard method)

Using the national standard method for COD determination is to add excess potassium dichromate standard solution to the water sample in a strong acid solution such as sulfuric acid-silver sulfate, and perform heating and refluxing for two hours, which is used to carry out sufficient reduction of the reducing substances in the water sample. oxidation. On the whole, the use of the national standard method to determine COD has the characteristics of convenient operation and relatively accurate measurement results, but it is worth noting that the use of the national standard method requires a large number of samples and a relatively large amount of reagents, which will also cause surrounding problems. The secondary pollution of the environment, coupled with the relatively long analysis time, and the high overall cost of the measurement, there are relatively large limitations on the use of this COD measurement technology.

When using the national standard method for COD measurement, in order to ensure accurate measurement quality, the following aspects should be paid attention to:

1. If there is a large amount of chloride (generally greater than 30mg/L) in the water sample, during the measurement process, because chloride ions can consume potassium dichromate, the resulting measurement error will affect the final result. Positive interference is formed, so it is necessary to add an appropriate amount of mercury sulfate powder during the test to eliminate the influence caused by chloride ions. During the test, the water sample needs to use potassium hydrogen phthalate and glucose to prepare a standard solution. ion interference, it is not necessary to add mercury sulfate solution to prevent contamination of water samples.

2. In the process of pipetting potassium dichromate, if the liquid discharge speed is too fast, a large amount of solution will remain in the internal position of the pipette, which will affect the accuracy of COD measurement. In the process of digestion, the rate of liquid discharge should be properly controlled and slowed down. In addition, it should be noted that the solution after digestion must be cooled to room temperature before dropping the indicator into the titration operation.

3. For the constant temperature heater, its temperature needs to be controlled within the range of 160-165°C, which can keep the solution in the digestion bottle in a slightly boiling state and prevent the volatile substances in the solution from overflowing from the upper part of the air condenser. , for the final determination of potassium dichromate, three significant figures should be retained uniformly.